The invention relates generally to the synthesis of organic compounds and, more specifically, to the synthesis of polyoxazolidones from bisanhydrohexitols.
Much effort is being currently directed into finding routes to new and useful materials, derived from biomass precursors. One valuable source is a class of sugar derivatives called bisanhydrohexitols. Such materials are usually water-loving and harmless and find widespread use in such areas as pharmaceuticals and food chemistry. They are derived from hexose sugars by reduction to the appropriate hexitols, which are then cyclo-dehydrated in the presence of mineral acids such as sulfuric acid. The most widely available bisanhydrohexitol is isosorbide, which can be obtained from sorbitol, in turn made from glucose which comes from biomass by fermentation reactions. Others are isomannide (from mannose) and isoidide (from idose). These three specific bisanhydrohexitols (also called isohexides) are particularly useful because they have a cis-bridged bicyclic structure containing two fused tetrahydrofuran rings. This stereochemistry lends itself well to the synthesis of useful structural polymers. Isosorbide is increasingly becoming used as an intermediate and raw material for polymers, additives and stabilizers.
It is known to use isosorbide as a diol co-monomer in polyesters such as polyethylene terephthalate (Adelman et alia, U.S. Pat. No. 6,656,577) and in polytrimethylene terephthalate (Adelman et alia, U.S. Pat. No. 7,049,390). The stiffness and rigidity of the isosorbide ring structure serves to raise the glass-rubber transition temperature (Tg) of these polyesters. Because isosorbide is a stiff functional material, other polymers such as polyurethanes and epoxy resins have been explored. One desirable reason for using isosorbide and its analogues is that they are harmless materials which are already used in medicinal polymers and certain pharmaceuticals and meet the FDA criteria for substances that are Generally Recognized as Safe (GRAS). An added advantage is that they may be used to replace materials such as bisphenol-A, which not only an intermediate derived from petroleum sources but one which is known to be a xenestrogen and is under increasing suspicion of being an endocrine disrupter. Traces of bisphenol-A are widely distributed throughout the environment and may enter the food chain, since epoxy resins containing bisphenol-A are frequently used in can coatings. Condensation of isosorbide or other isohexides with epichlorhydrin to make diglycidyl ethers is described by Zech and Le Maistre (U.S. Pat. No. 3,272,845) and by Morrison (U.S. Pat. No. 3,041,300). Recent work by East and coworkers has described high-performance cured epoxy-resins with excellent physical properties using glycidyl ethers derived from bisanhydrohexitols and cured with acid anhydrides or cyclic polyamines. Such cured epoxy resins are described in U.S. Patent Application No. 2008/0009599 (East et alia), which is incorporated herein in its entirety by this reference.
Polyoxazolidones are polymers made by curing epoxy resins with polyisosocyanates. Typical examples of the synthesis of such polymers have been taught by Koenig et alia in U.S. Pat. No. 5,112,932, and Narahara in U.S. Pat. No. 4,070,416. The curing mechanism is generally believed to be the addition and ring-opening of the three-membered epoxy ring onto the isocyanate group to form the five-membered oxazolidone ring, a reaction catalyzed by a wide variety of catalysts, such as quaternary ammonium salts and various organometallic compounds (see U.S. Pat. No. 4,066,628) as well as tertiary amines such as N-alkyl morpholines, alkylimidazoles and diazabicyclo 2,2,2-octane, (see U.S. Pat. No. 4,070,416). Polyoxazolidones are known to form highly crosslinked and rigid cured thermoset resins having a high solvent resistance. It is believed that the crosslinking is due to a combination of oxazolidone formation and trimerization of the isocyanate groups to form trifunctional isocyanurate linkages. According to Narahara the degree of formation of isocyanurates is favored by curing below 130° C. while the formation of oxazolidones is favored by curing above 130° C. The cured polymers have been found particularly useful for thermoset resins used as printed circuit board laminates, compression molded parts, insulated lacquers for coils and electric circuitry, where their solvent resistance and stiffness is used to great effect.
The electrical and electronic industries world wide have in recent years begun a wide-ranging campaign to improve their environmental record as exemplified by the abolition, at considerable expense, of lead based solders and their replacement with tin-silver solder alloys. By the same token, the replacement of thermoset polymers based upon materials such as bisanhydrohexitols rather then the environmentally unsound bisphenol-A based materials is a step in line with this philosophy.